The present invention relates to a system for injecting a fluid reductant into an exhaust system, a method of operating the system, and a flow guide for use in the system.
To reduce NOx levels in exhaust systems, notably diesel exhaust systems, it is known to inject a reductant into the exhaust gas upstream of a Selective Catalytic Reduction (SCR) module. The reductant converts NOx to nitrogen gas and water in the presence of the SCR. Typical reductants used are ammonia or urea, with urea generally preferred for safety and handling reasons. Urea is converted to ammonia through thermal decomposition.
The liquid reductant (for example aqueous urea) may be injected by a liquid-only dosing system, or by means of an air-assisted dosing system. Air-assisted dosing provides a smaller and more uniform droplet size through a centerline injection.
A problem with such reductant injection is that the mist of droplets from the injector tip may deposit on the wall of the exhaust pipe. This reduces the efficiency of the process and, in the case of urea and low-temperature operation, can lead to deposits which partly or completely block the exhaust pipe. A high concentration of droplets on or near the wall presents a significant challenge to mix and give an even distribution of the reductant in the exhaust gas at the SCR catalyst.
It has been proposed in U.S. 2010/0212292, to provide a feed connector through which the reductant is injected and which is provided with an inlet opening for generating additional gas flow along the inner wall of the feed connector to provide a gas barrier for preventing mist droplets from depositing on the inside wall of the feed connector or the exhaust pipe. WO 2011/106487 discloses a system for injecting a liquid reductant into an exhaust gas via an internal cone disposed in and generally parallel to the exhaust pipe. A reductant injector is in fluid communication with the cone. The cone has holes to permit inward flow of exhaust gas for creating a drag force on the injected liquid reductant. The drag force is said to increase the travel time of droplets from the injector to the SCR catalytic converter, allowing greater heating and consequently greater evaporation and/or thermal decomposition of the urea or other reductant.
Injecting aqueous urea reductant liquid on a bend in the exhaust pipe has a number of performance and practical benefits, including maximizing the downstream mixing length, providing more packaging flexibility and reducing injector temperature issues. However, placing the injector on the bend results in the majority of the spray following the main exhaust flow, particularly if the droplets are fine (as in air-assist injection systems). Even with high initial injection droplet velocity, the droplet inertia is small in comparison to the drag forces exerted by the main exhaust flow. The result of this is that the majority of droplets impinge on the wall of the exhaust pipe, losing any benefits of the small droplets for mixing and breakdown to ammonia (on evaporation and turbulent dispersion).